Three-tandem perovskite/silicon-based tandem solar cell

ABSTRACT

A three-tandem (3T) perovskite/silicon (PVT)-based tandem solar cell (TSC) includes an antireflection coating (ARC), a first transparent conductive oxide layer (TCO), a hole transport layer (HTL), a perovskite (PVT) layer, a second transparent conductive oxide layer (TCO), an electron transport layer (ETL), a plurality of buried contacts, a p-type Si layer, a p-type wafer-based homo-junction silicon solar cell, a n+ silicon layer, a back contact layer. The solar cell further includes a top sub-cell, a bottom sub-cell and a middle contact-based tandem. The top sub-cell includes the PVT layer. The bottom sub-cell includes the silicon solar cell. The middle contact-based tandem includes the second TCO layer to be used as the middle contact-based tandem, as well as a recombination layer for current collection. Further, a conduction and a valence band edge are employed at a front surface of the ETL.

BACKGROUND Technical Field

The present disclosure is directed to a solar cell, and particularly toa three-tandem (3T) perovskite/silicon (PVT)-based tandem solar cell.

Description of Related Art

The “background” description provided herein presents the disclosurecontext generally. Work of the presently named inventors, to the extentit is described in this background section, and aspects of thedescription that may not otherwise qualify as prior art at the time offiling, are neither expressly nor impliedly admitted as prior artagainst the present invention.

A solar cell utilizing an organic metal perovskite crystal material(perovskite-type solar cell) or other organic and/or inorganic hybrid(OIH) materials can provide high conversion efficiency. Perovskite(PVT)-based tandem solar cells (TSCs) are emerging as a leadingphotovoltaic (PV) technology and have the potential to cross theShockley-Queisser (S-Q) theoretical limit of a single-junction siliconsolar cell. However, 2-terminal (2T) and 4T PVT/Si TSCs have one or morelimitations, such as low power conversion efficiency (ii) and highmanufacturing costs. Furthermore, the 2T and 4T PVT/Si TSCs may includematerials that are inefficient solar radiation absorbers. Hence, thereis a need for a simple, an efficient, and an inexpensive solar cell thatmay overcome the limitations above.

Therefore, it is one object of the present disclosure to provide asimple and efficient solar cell with high power conversion efficiencyand low manufacturing costs.

SUMMARY

In an exemplary embodiment, a three-tandem (3T) perovskite/silicon(PVT)-based tandem solar cell (TSC) is described. The solar cellincludes an antireflection coating (ARC). The solar cell includes afirst transparent conductive oxide layer (TCO). The ARC is adjacent andabove the TCO. The solar cell further includes a hole transport layer(HTL). The TCO is adjacent and above the HTL. The solar cell furtherincludes a perovskite (PVT) layer. The HTL is adjacent and above the PVTlayer. The solar cell includes a second transparent conductive oxidelayer (TCO). The PVT layer is adjacent and above the second TCO. Thesolar cell further includes an electron transport layer (ETL) includinga porous silicon surface. The second TCO layer is adjacent and above theETL. The solar cell further includes a plurality of buried contactsincluding silicon nanowires and are disposed in and pass through the ETLinto the second TCO and into a p-type Si layer. The solar cell includesa p-type wafer-based homo-junction silicon solar cell with the buriedcontacts being in point contact configuration with the ETL. The ETL isadjacent and above the p-type silicon solar cell. The solar cell furtherincludes a n⁺ silicon layer. The p-type silicon solar cell is adjacentand above the n⁺ silicon layer. The solar cell includes a back contactlayer. The n⁺ silicon layer is adjacent and above the back contactlayer, which uses an internal thermal barrier. The three-tandem (3T)perovskite/silicon (PVT)-based tandem solar cell (TSC) has a topsub-cell, a bottom sub-cell and a middle contact-based tandem. The topsub-cell includes the PVT layer. The bottom sub-cell includes thesilicon solar cell. The middle contact-based tandem includes the secondTCO layer to be used as the middle contact-based tandem, as well as arecombination layer for current collection. A conduction and a valenceband edge are employed at a front surface of the ETL. The porous siliconsurface is directly grown on a surface of the silicon solar cell. Theband edge is tuned by varying a porosity of the porous silicon surface.

In some embodiments, the bottom sub-cell includes a CZ crystallinesilicon (CZ c-Si).

In some embodiments, the bottom sub-cell includes a multi-crystallinesilicon (mc-Si).

In some embodiments, the first and second TCO layers are fluorine orindium doped tin oxide (FTO or ITO).

In some embodiments, the first TCO layer is a hydrogenated indium oxide.

In some embodiments, the top sub-cell includes an organic or inorganicperovskite.

In some embodiments, the PVT layer includes one or more perovskiteshaving a formula ABX₃. Where A is a monovalent cation selected from thegroup consisting of methylammonium (MA⁺:CH₃NH₃ ⁺), formamidinium(FA⁺:HC(NH₂)₂ ⁺), cesium (Cs⁺), and rubidium (Rb⁺). B is a divalentmetal cation selected from the group consisting of Pb²⁺, Sn²⁺, and Ge²⁺,and X is a halide anion selected from the group consisting of Cl⁻, Br⁻,and I⁻.

In some embodiments, the porous silicon surface uses an opticalband-edge shifting property to directly extract light-generated chargecarriers.

In some embodiments, the bottom sub-cell contributes less than half of atotal efficiency of the TSC.

In some embodiments, the second TCO layer has a gridded metal contactdisposed adjacent to a surface of the second TCO layer.

In some embodiments, the p-type silicon solar cell has a resistivity offrom 1 (ohm-centimeters) Ω-cm to 10 Ω-cm.

In some embodiments, the p-type silicon solar cell has a thickness offrom 175 micrometers (μm) to 225 μm.

In some embodiments, the solar cell further includes at least one of aSiO₂ layer or Al₂O₃ layer on the porous silicon surface.

In some embodiments, the SiO₂ or Al₂O₃ layer is at most 10 nanometers(nm).

In some embodiments, the HTL includes a hole-transmitting material(HTM).

In some embodiments, the HTM is selected from the group consisting ofNiO, NiO:Cu, and WO₃.

The foregoing general description of the illustrative present disclosureand the following detailed description thereof are merely exemplaryaspects of the teachings of this disclosure and are not restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of this disclosure and many of theattendant advantages thereof will be readily obtained as the samebecomes better understood by reference to the following detaileddescription when considered in connection with the accompanyingdrawings, wherein:

FIG. 1 is a schematic drawing of a three-tandem (3T) perovskite/silicon(PVT)-based tandem solar cell (TSC), according to certain embodiments;

FIG. 2 is a schematic structure of a PVT having a generic form of ABX₃,according to certain embodiments;

FIG. 3 is a schematic diagram of a PVT/Si tandem structure, according tocertain embodiments;

FIG. 4 is a graph depicting a spectrum irradiance response of PVT/SiTSCs, according to certain embodiments;

FIG. 5A is a schematic drawing of a 4 terminal (T) tandem cell,according to certain embodiments;

FIG. 5B is a schematic drawing of a 2T tandem cell, according to certainembodiments;

FIG. 5C is a schematic drawing of a first 3T tandem cell, according tocertain embodiments;

FIG. 5D is a schematic drawing of a second 3T tandem cell, according tocertain embodiments;

FIG. 6 is a schematic view of mechanically stacked, a graphene-based 2TPVT/Si TSC obtained by applying pressure over a contact area ofsub-cells, according to certain embodiments;

FIG. 7A is a schematic diagram of a 3T PVT/Si integrated back contact(IBC) TSC along with an operational mechanism, according to certainembodiments;

FIG. 7B is a cross-sectional scanning electron microscope (SEM) image ofa top sub-cell, according to certain embodiments;

FIG. 8 is a schematic diagram of an exemplary PVT/Si TSC structure on ann-type Si wafer (c-Si or mc-Si) as a bottom sub-cell, according tocertain embodiments;

FIG. 9 is a schematic diagram of a proposed monolithic PVT/Si 3T tandemdevice concept for n-type wafer-based homo-junction silicon solar cell,according to certain embodiments;

FIG. 10 is a schematic diagram of an exemplary PVT/Si TSC structure on ap-type Si (c-Si or mc-Si) wafer as a bottom sub-cell, according tocertain embodiments; and

FIG. 11 is a schematic diagram of a proposed monolithic PVT/Si 3T tandemdevice concept for p-type wafer-based homo-junction silicon solar cell,according to certain embodiments.

DETAILED DESCRIPTION

In the drawings, like reference numerals designate identical orcorresponding parts throughout the several views. Further, as usedherein, the words “a,” “an” and the like generally carry a meaning of“one or more,” unless stated otherwise.

Furthermore, the terms “approximately,” “approximate,” “about,” andsimilar terms generally refer to ranges that include the identifiedvalue within a margin of 20%, 10%, or preferably 5%, and any valuesthere between.

Aspects of the present disclosure are directed towards a three-tandem(3T) perovskite/silicon (PVT)-based tandem solar cell (TSC), otherwisereferred to as the ‘solar cell’. A solar cell, or photovoltaic cell, isan electrical device that converts light energy directly intoelectricity by the photovoltaic effect, which is a physical and chemicalphenomenon. Experimental observations for the solar cell componentsdemonstrated a significant increase in power conversion efficiency (ii).In addition, the solar cell exhibits more extended service life at lowcosts, thereby circumventing the drawbacks such as high manufacturingcost and low power conversion properties of the prior art.

FIG. 1 refers to a schematic drawing of a three-tandem (3T)perovskite/silicon (PVT)-based tandem solar cell (TSC) 100. The solarcell 100 includes an anti-reflection coating (ARC) 102. The ARC 102reduces reflection and increases light absorption and performance of thesolar cell 100. In some embodiments, the ARC layer 102 may includesilicon dioxide. In some embodiments, the ARC 102 has a thickness offrom 60 micrometers (μm) to 120 μm, preferably 70 μm to 110 μm,preferably 80 μm to 100 μm, or 90 μm. In some embodiments, the ARC 102may further include titanium dioxide. In some embodiments, the ARC 102may include LiF or MgF₂. In some embodiments, the ARC 102 may includeMgF₇, SiN_(x), SiO₂, TiZrO₂, ZnS, SiN CeO₂, ITO, Si₃N₄, ZnO, TiO₂, afluoropolymer (PTFE, PVdF, PHFP, etc.), spirooxazine-doped polystyrene,vinyltrimethoxy silane films, or AlN wherein x may be any integer from 1to 10.

The solar cell 100 includes a first transparent conductive oxide layer(TCO) 104. The first TCO layer 104 is an electrically conductivematerial with comparably low absorption of light. The first TCO layer104 may include a chemical formula of A_(X)B_(Y), where A is a metal, Bis a non-metal such as oxygen, X and Y are atoms of the correspondingelements. The formula of the first TCO layer 104 may further be changedto a formula A_(y)B_(z):D on doping. The ARC 102 is adjacent and abovethe first TCO layer 104 and directly contacts the first TCO layer 104.In some embodiments, the refractive index of the ARC 102 is lower than arefractive index of the first TCO layer 104. In some embodiments, theARC 102 is a material with an index of refraction of 1.23±0.01, 0.02,0.03, 0.04, 0.05, 0.06, 0.07, 0.075, 0.08, 0.09, 0.1, 0.11, 0.125,0.133, 0.14, or 0.15, or some range including any of these endpoints,different from the layer immediately beneath it, the first TCO layer104. In some embodiments, the first TCO layer 104 has a thickness offrom 60 micrometers (μm) to 120 μm, preferably 70 μm to 110 μm,preferably 80 μm to 100 μm, or 90 μm.

A dopant may be added to the first TCO layer 104. For example, when zincoxide is used for the first TCO layer 104, examples of the dopant mayinclude aluminum, gallium, boron, silicon, and carbon. When indium oxideis used for the first TCO layer 104, examples of the dopant may includezinc, tin, titanium, tungsten, molybdenum, and silicon. When tin oxideis used for the first TCO layer 104, examples of the dopant may includefluorine. In some embodiments, the first TCO layer 104 layer may be ahydrogenated indium oxide. In some embodiments, the first TCO layer 104is fabricated of indium tin oxide (ITO), indium zinc oxide (IZO),aluminum-doped zinc oxide (AZO), gallium-doped zinc oxide (GZO), indiumcerium oxide (ICO), indium tungsten oxide (IWO), zinc indium tin oxide(ZITO), zinc indium oxide (ZIO), zinc tin oxide (ZTO), GITO (galliumindium tin oxide), gallium indium oxide (GIO), gallium zinc oxide (GZO),aluminum-doped zinc oxide (AZO), fluorinated tin oxide (FTO), ZnO, orand/or indium-doped cadmium oxide (ICO), and can be deposited viathermal evaporation or sputtering.

The solar cell 100 further includes a hole transport layer (HTL) 106. Insome embodiments, the HTL 106 may include an organic HTL material. Insome embodiments, the HTL 106 may include an inorganic HTL material. Insome embodiments, the HTL 106 may be selected from the group of1,3-bis(N-carbazolyl)benzene, 4,4′-bis(N-carbazolyl)-1,1′-biphenyl,2,6-bis(9H-carbazol-9-yl)pyridine, 1,4-bis(diphenylamino)benzene,4,4′-bis(3-ethyl-N-carbazolyl)-1,1′-biphenyl,N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine,(E,E)-1,4-bis[2-[4-[N,N-bis(4-methoxyphenyl)amino]phenyl]vinyl]benzene(TOP-HTM-al),(E,E,E,E)-4,4′,4″,4′-[benzene-1,2,4,5-tetrayltetrakis(ethene-2,1-diyl)]tetrakis[N,N-bis(4-methoxyphenyl)aniline](TOP-HTM-α2),copper(II) phthalocyanine, cuprous thiocyanate, copper indium sulfide,cuprous iodide,4,4′-cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine],4-(dibenzylamino)benzaldehyde-N,N-diphenylhydrazone,9,9′-(2,2′-dimethyl[1,1′-biphenyl]-4,4′-diyl)bis-9H-carbazole,2,2′-dimethyl-N,N′-di-[(1-naphthyl)-N,N′-diphenyl]-1,1′-biphenyl-4,4′-diamine,9,9-dimethyl-N,N′-di(1-naphthyl)-N,N′-diphenyl-9H-fluorene-2,7-diamine,N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine,N,N′-diphenyl-N,N′-bis-[4-(phenyl-m-tolylamino)phenyl]biphenyl-4,4′-diamine,N,N′-diphenyl-N,N′-di-p-tolylbenzene-1,4-diamine,dipyrazino[2,3-f:2′,3′-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile,N⁴,N⁴′-bis[4-[bis(3-methylphenyl)amino]phenyl]-N⁴,N⁴′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(DNTPD), 3-(4,6-diphenyl-1,3,5-triazin-2-yl)-9-phenyl-9H-carbazole(DPTPCz),9-(2-ethylhexyl)-N,N,N,N-tetrakis((4-methoxyphenyl)-9H-carbazole-2,7-diamine)(EH44), indium(III) phthalocyanine chloride, lead phthalocyanine,poly(copper phthalocyanine), poly(N-ethyl-2-vinylcarbazole),poly-4-butyl-N,N-diphenylaniline (TPD), poly(9-vinylcarbazole),poly(1-vinylnaphthalene),2,8-bis(diphenylphosphineoxide)dibenzofuran(PPF),poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine](PTAA),N₂,N₂,N₂′,N₂′,N₇,N₇,N₇′,N₇′-octakis(4-methoxyphenyl)-9,9′-spirobi[9H]-fluorene]-2,2′,7,7′-tetramine(Spiro-MeOTAD, also sold as SHT-263 Solarpur® HTM),spiro[9H-fluorene-9,9′-[9H]xanthene]-2,7-diamine,spiro[9H-fluorene-9,9′-[9H]xanthene]-2,2′,7,7′-tetramine,2,4,6-tris(3-(carbazol-9-yl)phenyl)triazine(TCPZ),N,N,N′,N′-tetrakis(4-methoxyphenyl) benzidine,N,N,N′,N′-tetrakis(3-methylphenyl)-3,3′-dimethylbenzidine,N,N,N′,N′-tetrakis(2-naphthyl)benzidine,tetra-N-phenylbenzidine,N,N,N′,N′-tetraphenylnapthalene-2,6-diamine,poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4′-(N-(4-sec-butylphenyl)diphenylamine)](TFB), tin(IV) 2,3-naphthalocyanine dichloride, titanyl phthalocyanine,titanium oxide phthalocyanine, tris(4-carbazoyl-9-ylphenyl)amine,tris[4-(diethylamino)phenyl]amine, 1,3,5-tris(diphenylamino)benzene,1,3,5-tris(2-(9-ethylcabazyl-3)ethylene)benzene,1,3,5-tris[(3-methylphenyl)phenylamino]benzene,4,4′,4″-Tris[2-naphthyl(phenyl)amino]triphenylamine,4,4′,4″-tris[phenyl(m-tolyl)amino]-triphenylamine,vanadyl phthalocyanine, zinc phthalocyanine, or combinations of any ofthese. In some embodiment, the HTL 106 may include one or more additivesto reduce resistivity. The one or more additives may include solidadditives such as Li-bis(trifluoromethanesulfonyl)imide (Li-TFSI),liquid additives such as 4-tert-butylpyridine (tBP), and metal complexesincluding Co. In some embodiments, the HTL layer 106 has a thickness offrom 60 micrometers (μm) to 120 μm, preferably 70 μm to 110 μm,preferably 80 μm to 100 μm, or 90 μm.

The first TCO layer 104 is adjacent and above the HTL 106. In someembodiments, the first TCO layer 104 directly contacts the HTL 106. Insome embodiments, the HTL 106 includes a hole-transmitting material(HTM). In some embodiments, the HTM is selected from the groupconsisting of NiO, NiO:Cu, and WO₃, and further can be selected from thegeneric list of NiO_(x), NiO:Cu_(x), or WO_(x), wherein x is an integerbetween 1 and 10. The solar cell 100 further includes a perovskite (PVT)layer 108. The HTL 106 is adjacent and above the PVT layer 108. In someembodiments, the HTL 106 directly contacts the PVT layer 108. In someembodiments, the PVT layer 108 includes one or more perovskites having aformula ABX₃. Where A is a monovalent cation selected from the groupconsisting of methylammonium (MA⁺:CH₃NH₃ ⁺), formamidinium (FA⁺:HC(NH₂)₂⁺), cesium (Cs⁺), and rubidium (Rb⁺), B is a divalent metal cationselected from the group consisting of Pb²⁺, Sn²⁺, and Ge²⁺, and X is ahalide anion selected from the group consisting of Cl⁻, Br⁻, and I⁻. Inalternate embodiments, the cation ‘A’ is inorganic, the cation can beselected from the group consisting of Ag+, Li+, Na+, K+, Be2+, Mg2+,Ca2+, Pb2+, Sr2+, Ba2+, Fe2+, Sc3+, Y3+, and La3+. The cation can beused as a single or multiple ion (e.g. (Mg, Fe)SiO3), YBaCuO3). In someembodiments, the PVT layer 108 has a thickness of from 120 micrometers(μm) to 240 μm, preferably 130 μm to 230 μm, preferably 140 μm to 220μm, preferably 150 μm to 210 μm, preferably 160 μm to 200 μm, preferably170 μm to 190 μm, or 180 μm.

In some embodiments, the formula ABX₃ may also be interchangeablyreferred to as the formula RNH₃MX₃, where R is an alkyl group,preferably an alkyl group having 1 to 10 carbon atoms, and particularlypreferably a methyl group, M is same as that of the B of the formulaABX₃. Non-limiting examples of an alkyl group include ethyl, propyl,isopropyl, butyl, isobutyl, sec-butyl, tert-butyl, and the higherhomologs and isomers.

The solar cell 100 includes a second transparent conductive oxide layer(TCO) 110. The PVT layer 108 is adjacent and above the second TCO layer110. In some embodiments, the PVT layer 108 directly contacts the secondTCO layer 110. The second TCO layer 110 may include a chemical formulaof A_(X)B_(Y), where A is a metal, B is a non-metal such as oxygen, Xand Y are atoms of the corresponding elements. The formula of the secondTCO layer 110 may further be changed to a formula A_(y)B_(z):D ondoping. A dopant may be added to the second TCO layer 110.

For example, when zinc oxide is used for the second TCO layer 110,examples of the dopant may include aluminum, gallium, boron, silicon,and carbon. When indium oxide is used for the second TCO layer 110,examples of the dopant may include zinc, tin, titanium, tungsten,molybdenum, and silicon. When tin oxide is used for the second TCO layer110, examples of the dopant may include fluorine. In some embodiments,the second TCO layer 110 layer may be a hydrogenated indium oxide. Insome embodiments, the second TCO layer 110 has a thickness of from 50micrometers (μm) to 100 μm, preferably 57.5 μm to 92.5 μm, preferably 65μm to 85 μm, or 75 μm. In some embodiments, the second TCO layer 110 hasa gridded metal contact disposed adjacent to a surface of the second TCOlayer 110. In some embodiments, the gridded metal contact is fabricatedof Ag, Sm, Sc, Gd, Er, or other precious metals. The solar cell 100further includes an electron transport layer (ETL) 112. In someembodiments, the ETL 112 may be selected from one or more materials suchas graphene, graphite, graphene oxide, zirconia, titanium oxide, zincoxide, niobium oxide, zirconium oxide, and aluminum oxide. In someembodiments, the ETL 112 may include a donor. For example, when titaniumoxide is used for the ETL 112, examples of the donor may includeyttrium, europium, and terbium. In some embodiments, the ETL layer 112has a thickness of from 60 micrometers (μm) to 120 μm, preferably 70 μmto 110 μm, preferably 80 μm to 100 μm, or 90 μm.

The ETL 112 may be a dense layer having a smooth structure, or porouslayer having a porous structure. When the ETL 112 has a porousstructure, the micropore size is preferably on the nanoscale, in a rangefrom 1 nm to 2 nm, preferably 1.1 nm to 1.9 nm, preferably 1.2 nm to 1.8nm, preferably 1.3 nm to 1.7 nm, preferably 1.4 nm to 1.6 nm, or 1.5 nm.In some embodiments, the ETL has a mesopore size ranging from 2 nm to 50nm, preferably 5 nm to 45 nm, preferably 10 nm to 40 nm, preferably 15nm to 35 nm, preferably 20 nm to 30 nm, or 25 nm. In some embodiments,the ETL 112 includes a porous silicon surface. In some embodiments, theporous silicon surface uses an optical band-edge shifting property todirectly extract light-generated charge carriers. The shifting propertyrefers to optical recombination, and/or resistive losses that are to bereduced with the present disclosure. Aspects of the present disclosureenhance the open circuit voltage (V_(oc)), e.g., via surfacemodification of silicon, optionally further enhancing the open circuitvoltage (V_(oc)) of the tandem cell. The porous silicon surfaceincreases the active surface area of the light absorbing layer toimprove the collectiveness of electrons by the ETL 112, thus poroussilicon surface can directly produce current via light absorption. Insome embodiments, The ETC layer 112 may comprises at least 90 wt. %p⁺-porous silicon, relative to total electron transport layer weight. Insome embodiments, the ETC layer 112 comprises no filler. In someembodiments, the ETC layer 112 may comprise no perovskite materialbeyond a depth of 10% of an ETC layer thickness. In some embodiments,the porosities of the porous silicon in the ETC layer 112 may involve,e.g., at least 1, 2.5, 5, 7.5, 10, 12.5, 15, 17.5, 20, 22.5, 25, 27.5,30, 35, 40, 45, or 50 vol. % (void) and/or up to 95, 90, 85, 80, 75, 70,65, 60, 55, 50, 45, 40, 35, 30, or 25%. In some embodiments, the shiftin the energy levels of the conduction band minimum (CBM) and thevalence band maximum (VBM) of porous silicon (PS) are in an approximateratio of 1/2.6, e.g., 1 to at least 2.25, 2.3, 2.33, 2.35, 2.4, 2.45,2.5, 2.525, 2.55, 2.575, 2.6, 2.625, or 2.65 and/or up to 2.9, 2.85,2.8, 2.775, 2.75, 2.725, 2.7, 2.675, 2.65, 2.625, 2.6, 2.575, or 2.55.Due to the band mismatch of porous silicon with PVT 108, minimalinterfacial layers are required to transport electrons (or holes) fromthe porous silicon (or PVT 108) to PVT 108 (or porous silicon), rangingfrom 1 to 3 interfacial layers, or 2 layers, thereby reducing materialand processing costs.

The second TCO layer 110 is adjacent and above the ETL 112. In someembodiments, the second TCO layer 110 is in direct contact with the ETL112. The solar cell 100 further includes a plurality of buried contacts114 in the form of silicon nanowires and are disposed in and passthrough the ETL 112 into the second TCO layer 110 and into a p-type Silayer. In some embodiments, the buried contacts 114 in the form ofsilicon nanowires have a diameter of from 0.5 μm to 1.5 μm, preferably0.6 μm to 1.4 μm, preferably 0.7 μm to 1.3 μm, preferably 0.8 to 1.2 μm,preferably 0.9 μm to 1.1 μm, or 1 μm. In alternate embodiments, thenanowires are formed of titanium nitride, germanium, or manganese. Insome embodiments, the buried contacts 114 have a height ranging from 60micrometers (μm) to 120 μm, preferably 70 μm to 110 μm, preferably 80 μmto 100 μm, or 90 μm. In some embodiments, the buried contacts 114penetrate to a depth ranging from 2 micrometers (μm) to 20 μm into thesecond TCO layer 110, preferably 4 μm to 18 μm, preferably 6 μm to 14μm, preferably 8 μm to 12 μm, or 10 μm. In some embodiments, the firstTCO layer 104 and the second TCO layer 110 are fluorine or indium dopedtin oxide (FTO or ITO). In some embodiments, the first TCO layer 104 andthe second TCO layer 110 are doped using zinc oxide. The solar cell 100includes a p-type wafer-based homo-junction silicon solar cell, alsoreferred to as the p-type silicon solar cell 115. The buried contact 114is in point contact configuration with the ETL 112. In some embodiments,the buried contacts 114 penetrate to a depth ranging from 2 micrometers(μm) to 20 μm into the p-type silicon solar cell 115, preferably 4 μm to18 μm, preferably 6 μm to 14 μm, preferably 8 μm to 12 μm, or 10 μm. Insome embodiments, the buried contacts 114 penetrate the entire thicknessof the ETL 112. In alternate embodiments, the buried contacts 114penetrate between 70% to 95% of the thickness of the ETL 112, preferably72.5% to 92.5%, preferably 75% to 90%, preferably 77.5% to 87.5%,preferably 80% to 85%, or 82.5%.

The ETL 112 is adjacent and above the p-type silicon solar cell 115. Insome embodiments, the ETL 112 is in direct contact with the solar cell115. In some embodiments, p-type silicon solar cell 115 has a thicknessof from 150 micrometers (μm) to 300 μm, preferably 175 μm to 275 μm,preferably 200 μm to 250 μm, or 225 μm. In preferred embodiments, thep-type silicon solar cell has a thickness of from 175 micrometers (μm)to 225 μm, preferably 180 μm to 220 μm, preferably 185 μm to 215 μm,preferably 190 μm to 210 μm, preferably 195 μm to 205 μm, or 200 μm. Insome embodiments, the p-type silicon solar cell 115 has a resistivity offrom 1 (ohm-centimeters) Ω-cm to 10 Ω-cm, preferably 2 Ω-cm to 9 Ω-cm,preferably 3 Ω-cm to 8 Ω-cm, preferably 4 Ω-cm to 7 Ω-cm, preferably 5Ω-cm to 6 Ω-cm, or 5.5 Ω-cm. In some embodiments, the resistivity of thep-type silicon solar cell 115 may lie in a range of about 3 Ω-cm toabout 8 Ω-cm, preferably 3.5 Ω-cm to 7.5 Ω-cm, preferably 4 Ω-cm to 7Ω-cm, preferably 4.5 Ω-cm to 6.5 Ω-cm, preferably 5 Ω-cm to 6 Ω-cm, or5.5 Ω-cm. The p-type cell layer 115 may be made for example of one amongthe following elements: Nickel oxide “NiO_(x)”, Mobyldenum oxide“MoO_(x)”, Tungsten oxide “WO_(x)”,2,2′,7,7′-Tetrakis-(N,N-di-4-methoxyphenylamino)-9,9′-spirobifluorene,poly(triarylamine) “PTAA”, poly(3-hexylthiophene) “P3HT”,poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate)“PEDOT:PSS”, Copper(I) thiocyanate “CuSCN”, Cobalt oxide “CoO_(x)”,Chromium oxide “CrO_(x)”, Copper(I) iodide “CuI”, Copper sulfide “CuS”,Copper oxide “CuO_(x)”, or Vanadium oxide “Vo_(x)”, wherein x is aninteger between 1 and 10.

The solar cell 100 further includes a n⁺ silicon layer 116. The p-typesilicon solar cell 115 is adjacent and above the n⁺ silicon layer 116.In some embodiments, the solar cell 115 is in direct contact with thesilicon layer 116. In some embodiments, p-type silicon solar cell 115has a first polarity that is different from n⁺ silicon layer 116 whichhas a second polarity. In some embodiments, the n⁺ silicon layer 116 hasa thickness of from 60 micrometers (μm) to 120 μm, preferably 70 μm to110 μm, preferably 80 μm to 100 μm, or 90 μm. The solar cell 100includes a back contact layer 118. In some embodiments, the n-type layer116 may be made for example of one among the following elements: Tinoxide “SnO”, Titanliun oxide “TiO_(x)”, Zinc oxide “ZnO_(x)”, carbon,C₆₀ and derivatives, Zirconia “ZrO_(x)”, graphite, graphene, orgrapheneoxide “rGO”, wherein x is an integer between 1 and 10. The n⁺ siliconlayer 116 is adjacent and above the back contact layer 118, which usesan internal thermal barrier. The internal thermal layer can be definedas a layer that prevents heat transfer between neighboring layers oradjacent ambient air, limiting vast gradient temperature differencesthat may improve the material protection. In some embodiments, the backcontact layer 118 has a polyurethane foam thermal layer sprayed onto it,ranging from 0.5 μm to 1.5 μm in thickness, preferably 0.6 μm to 1.4 μm,preferably 0.7 μm to 1.3 μm, preferably 0.8 to 1.2 μm, preferably 0.9 μmto 1.1 μm, or 1 μm. In some embodiments, the silicon layer 116 is indirect contact with the back contact layer 118. In some embodiments, theback contact layer 118 has a thickness of from 60 micrometers (μm) to120 μm, preferably 70 μm to 110 μm, preferably 80 μm to 100 μm, or 90μm. The solar cell 100 includes a back contact layer 118. In someembodiments, the back contact layer is made of aluminum, titanium, orsilver. In alternate embodiments, the back contact layer 118 comprises apercentage of silicon and can be obtained by phase vapor deposition(PVD).

The three-tandem (3T) perovskite/silicon (PVT)-based tandem solar cell(TSC) 100 has a top sub-cell (A), a bottom sub-cell (B), and a middlecontact-based tandem (C). The top sub-cell includes the PVT layer. Insome embodiments, the top sub-cell includes an organic or inorganicperovskite.

The bottom sub-cell includes the silicon solar cell. In someembodiments, the bottom sub-cell includes a Czochralski (CZ) crystallinesilicon (CZ c-Si). In some embodiments, the c-Si may be asingle-crystalline, or microcrystalline. In some embodiments, the bottomsub-cell includes a multi-crystalline silicon (mc-Si). In someembodiments, the bottom sub-cell may include amorphous silicon (a-Si).In some embodiments, the bottom sub-cell contributes less than half of atotal efficiency of the TSC.

The middle contact-based tandem includes the second TCO layer 110 to beused as the middle contact-based tandem, as well as a recombinationlayer for current collection. A conduction and a valence band edge areemployed at a front surface of the ETL 112. The porous silicon surfaceis directly grown on a surface of the silicon solar cell. The band edgeis tuned by varying a porosity of the porous silicon surface. The poroussilicon layer can be directly grown or tuned on the surface of thesilicon via electrochemical etching or chemical etching, so that nointerface exists between the porous silicon surface and the second TCOlayer, for easier charge transportation. In some embodiments, theelectrochemical etching includes a single compartment Teflon cell usinga two-electrode arrangement. A silicon wafer is the working electrode,while Pt metal wire or foil acts as the counter electrode. Theelectrolyte is of a mixture of hydrofluoric acid (HF) and ethanol(C₂H₅OH) (or HF, ethanol, and deionized water (DIW)) in various ratios,preferably 1:2 to 1:20 by volume, preferably 1:4 to 1:18, preferably 1:6to 1:16, preferably 1:8 to 1:14, or 1:10. By applying various currentdensities, 1 to 100 mA/cm², preferably 10 to 90 mA/cm², preferably 20 to80 mA/cm², preferably 30 to 70 mA/cm², preferably 40 to 60 mA/cm², or 50mA/cm², for 5 s to 1000 s, preferably 100 s to 900 s, preferably 200 sto 800 s, preferably 300 s to 700 s, preferably 400 s to 600 s, or 500s. In a preferred embodiment, the current density ranges from 2 to 10mA/cm² for 5 s to 20 s. In some embodiments, the chemical etchingincludes dipping a silicon wafer in HF, HNO₃, and DIW using variouscompositions by volume (ratios), for various etching times, preferably 2s to 60 s, preferably 5 s to 55 s, preferably 10 s to 50 s, preferably15 s to 45 s, preferably 20 s to 40 s, preferably 25 s to 35 s, or 30 s.In this chemical etching process, HNO₃ oxidizes the silicon surface toSiO₂. In alternate chemical etching processes, silver nanoparticles(NPs) are uniformly deposited onto the porous substrate using an aqueoussolution of AgNO₃ and HF. The AgNO₃ concentration ranges from 0.01 to0.04 M, preferably 0.0125 M to 0.0375 M, preferably 0.015 M to 0.035 M,preferably 0.0175 M to 0.0325 M, preferably 0.02 M to 0.03 M, preferably0.0225 M to 0.0275 M, or 0.025 M and HF concentration ranges from 1-10M, preferably 2 M to 9 M, preferably 3 M to 8 M, preferably 4 M to 7 M,preferably 5 M to 6 M, or 5.5 M. In the etching deposition process, theAg ions capture electrons from Si and reduce them to Ag metal NPs. Thenanoparticles are deposited on silicon surfaces. In some embodiments,the etching was performed at room temperature in a mixed solution ofDIW, HF (4.6 M), and H₂O₂ (0.5 M) to obtain a porous silicon surface fora short etching time 1 to 10 s, preferably 2 to 9 s, preferably 3 to 8s, preferably 4 to 7 s, or 5 s. After the electroless etching, thesamples are dipped in concentrated HNO₃ for 10 min to completely removethe remaining Ag NPs, then rinsed in DIW, and dried in air.

In some embodiments, the solar cell 100 further includes at least one ofa SiO₂ layer or Al₂O₃ layer on the porous silicon surface. In someembodiments, the SiO₂ or Al₂O₃ layer is at most 10 nanometers (nm). Insome embodiments, the SiO₂ or Al₂O₃ layer is 8 nm.

EXAMPLES

The following examples describe and demonstrate exemplary embodiments ofthe solar cell described herein. The examples are provided solely forthe purpose of illustration and are not to be construed as limitationsof the present disclosure, as many variations thereof are possiblewithout departing from the spirit and scope of the present disclosure.

Multijunction TSC

Various solar cells are integrated into a tandem architecture, alsoreferred to as the multi-junction (MJ) solar cells. The MJ solar cellsinclude two or more single-junction (SJ) solar cells, including multipleabsorber layers with a complementary absorption range. Such a solarconcept, which separates the absorption of a polychromatic solarspectrum into several bandgaps, is referred to as the TSCs. The TSCsincrease power conversion efficiency (f) beyond the S-Q limit. In someembodiments, the multi-junction solar cells based on III-Vsemiconductors which are highly efficient and up to five differentabsorber layers are integrated into a single device. Typically, a doublejunction TSC is composed of two sub-cells, having a front cell, alsoreferred to as the top sub-cell of a bandgap (D 1.5-1.9 eV), and a rearcell, also referred to as the bottom sub-cell of a bandgap (Q 0.9-1.3eV). The top sub-cell absorbs high-energy photons and transmitslow-energy photons which are absorbed by the bottom sub-cell. Therefore,compared to single-junction solar cells, the TSCs allow maximumabsorption of the solar spectrum by minimizing the thermal energy lossesof photogenerated carriers. In some embodiments, the MJ solar cells mayinclude a monolithic MJ solar cell in which two or more SJ solar cellsare electrically series-connected via a tunnel junction (TJ) or arecombination layer (RL). The monolithic MJ solar cell has twoterminals. In some embodiments, the MJ solar cells may includemechanically stacked MJ solar cells in which two or more SJ solar cellsare mechanically stacked (not internally connected via TJ). Themechanically stacked MJ solar cells have multi-terminals (2×number of SJsub-cells).

Perovskite-Based TSCs

FIG. 2 refers to a schematic structure of a PVT having a generic form ofABX₃, where A⁺ is a monovalent cation such as methylammonium (MA⁺:CH₃NH₃⁺), formamidinium (FA⁺:HC(NH₂)₂ ⁺), cesium (Cs⁺), rubidium (Rb⁺) orcorresponding alloys. B²⁺ is a divalent metal cation that includes Pb²⁺,Sn²⁺, or Ge²⁺ or corresponding mixtures, and X⁻ is a halide anion suchas Cl⁻, Br⁻, I⁻ or corresponding combinations.

FIG. 3 refers to a schematic diagram of a PVT/Si tandem structure 300.The PVT/Si tandem structure 300 includes a TCO layer 302, a PVT topsub-cell 304, an interlayer 306, a bottom silicon sub-cell 308, and acathode 310. In PSCs, an ETL, and a HTL are used on either side of a PVTlayer to extract electrons and holes, respectively. The PVT layerabsorbs the solar spectrum only up to ˜800 nm wavelength (λ) and has astability issue against UV radiation, temperature, and moisture.Therefore, to utilize solar spectrum beyond 800 nm, hence, the tandemconfiguration of PVTs had been developed in combination with Si andcopper indium gallium selenide (CIGS). FIG. 3 shows that low wavelengthphotons are absorbed by the PVT top sub-cell 304, while the bottomsilicon sub-cell 308 absorbs lower energy photons. FIG. 4 is a graphdepicting a spectrum irradiance response of PVT/Si TSCs. The graph showsAM 1.5 standard spectrum 402, a PVT spectral irradiance 404, and asilicon spectral irradiance 406.

A PVT-based TSC was fabricated using Kesterite Cu₂ZnSn(S, Se)₄ as abottom sub-cell and methylammonium lead triiodide (MAPbI₃) as a topsub-cell and achieved a η of 4.6%. In some embodiments, PVT/Si,PVT/CIGS, and PVT/PVT (all-PVTs) are developed. In the PVT/Si, PVT/CIGS,PVT solar cell is used as a front sub-cell which can be attributed tothe semi-transparent nature of PVT films. However, in the PVT/PVT tandemarchitecture, front and rear cells are based on PVT materials. Known S-Qlimits of two-junction TSCs based on the PVT/Si, PVT/CIGS, and PVT/PVTare 44.3%, 44.1%, and 42.1%, respectively. PVT/Si and all-PVTs TSCs wereefficiently developed, and efficiencies of >29% had been achieved formonolithic PVT/c-Si, 28.2% for PVT/Si-heterostructure, and 25.9% forPVT/CIGS TSCs. Known theoretical η of PVT/c-Si TSCs is 45.3%, and theenergy conversion efficiency of practical devices is significantly lowerthan the theoretical η which can be attributed to various optical andelectrical losses such as parasitic photon absorption, excitondissociation, nonradiative recombination.

All-PVT TSC devices could be made on flexible and lightweight substratesvia a cost-effective solution process. However, all solution process wasnot feasible for PVT/Si and PVT/CIGS TSCs. First, 2-terminal (2T)all-PVTs TSCs were fabricated using two sub-cells of MAPbI₃ with anefficiency known to be □7%. The low device performance was attributed tothe unavailability of low bandgap PVT materials. However, thedevelopment of mixed Pb—Sn-based narrow bandgap (1.18-1.3 eV) PVTs hasenabled the fabrication of all-PVTs TSCs, in combination with Pb-basedwide-bandgap PVT, and efficiencies over 25% have been already achieved.

Device Architecture

FIGS. 5A-5D shows schematic diagrams of various double-junction TSCarchitectures such as 2T, 3T, and 4T based on electrical connections toextract current from top and bottom sub-cells. In general, the PVT-basedTSCs include 2T or 4T configurations. In FIG. 5A, if top sub-cell 502and bottom sub-cell 504 function independently (current is extractedseparately from the top sub-cell 502 and bottom sub-cell 504), then fourterminals (A-D) are needed for current extraction. The resultingstructure is referred to as a 4T tandem cell 500.

Further, as shown in FIG. 5B, if the top sub-cell 508 and bottomsub-cell 510 are monolithically interconnected, only two terminals, ‘E’and ‘F,’ are required to extract the current. The resulting tandemstructure is 2T tandem cell 560. In some embodiments, currents of twosub-cells of 2T tandems are matched for maximum output power (P_(m)).

In a 4T tandem cell, sub-cells are mechanically stacked or coupled witha spectrum-splitting dichromatic mirror. With extra transparentconductive oxide (TCO) layers, intricate interconnections, and separateinverters, the fabrication of the 4T configuration is expensive,producing low energy yield (E_(Y)) and higher losses. Conversely, in a2T tandem cell, the sub-cells are fabricated successively on a singlesubstrate and connected via an interconnection layer. The 2T cellmitigates optical and electrical losses which can be attributed to extraelectrodes and minimizes the manufacturing cost. Furthermore, the 2Tcell avoids lateral current flow through the TCO layer sandwichedbetween the sub-cells. Thus, only one inverter is used. However, theresultant output current is restricted by the sub-cell generating thelesser current. Hence, an optimal design may produce a maximum currentby matching the short circuit current density (J_(sc)) value of both thesub-cells. Several methods such as variation of illumination intensity,environmental conditions (such as wind speed, temperature, dust), and anangle of incident light during the daytime may lead to a mismatch of thecurrent of both the sub-cells, which may further reduce an output power(P_(m)) of 2T tandem devices. Thus, the rated value of the 2T-based TSCscannot be achieved in practical application. To overcome such issues, anew concept was introduced, in which a common connection is introducedbetween sub-cells (FIG. 5C and FIG. 5D).

FIG. 5C shows a schematic drawing of a first 3T tandem cell 580. Thefirst 3T tandem cell 580 includes a first sub-cell 582 and a secondsub-cell 584. The first sub-cell 582 includes a first terminal ‘A’ and asecond terminal ‘B’. The second sub-cell 584 consists of a top terminal‘C’ and a bottom terminal ‘D’. The second terminal ‘B’ and top terminal‘C’ work as a single unit ‘E’. The second terminal ‘B’ and top terminal‘C’ are collectively referred to as the single term ‘intermediateterminal’ ‘E’. FIG. 5D shows a schematic drawing of a second 3T tandemcell 590. The second 3T tandem cell 590 includes a first sub-cell 592and a second sub-cell 594. The first sub-cell 592 includes a firstterminal, ‘M’. The second sub-cell 594 includes a second terminal ‘N’and a third terminal ‘O’. In a 3T tandem configuration, current matchingof sub-cells is not essential. In 3T devices, both sub-cells areindependently operated in a monolithic structure on a single substrate(either in the form of middle contact or integrated back contact). 3Ttandem architecture resolves the issue associated with the currentmismatch in 2T and higher optical losses and manufacturing cost of 4T.

2T Tandem Architecture

In 2T TSCs, the resultant J_(sc) value is lower than the J_(sc) value ofindividual sub-cells. However, open-circuit voltage (V_(oc)) adds up. Abifacial Si hetero-junction (SHJ)/PVT TSC decouples limitationsassociated with 4T PSCs and provides a state-of-the-art solution toachieve a high η via utilizing scattered light from ground. In someembodiments, sub-cells exhibit η of 33% in a bifacial configuration,exceeding that of a bifacial SJH with an intrinsic thin layer (HIT) atan albedo reflection. The present structure is similar to thetraditional 2T TSC, but without a back contact, which leads to anenhancement in the η through harvesting reflected light from the ground.In some embodiments, the optical band gap (E_(g)) (˜1.55 eV) of a topPVT sub-cell is considerably lower than the desired E_(g) (˜1.75 eV). Inthe present configuration, the top PVT sub-cell absorbs a number ofphotons that the bottom sub-cell would not be able to generate adequateJ_(sc), which results in a η quashing. Therefore, tuning of the PVTlayer thickness is crucial for the desired design of a TSC with minimumJ_(sc) mismatch.

Properties and Influence of Tunnel Junction (TJ)

In some embodiments, a monolithic combination involves electricalcoupling between top and bottom sub-cells. Moreover, tailoringabsorption ranges of top and bottom sub-cells may be essential. In someembodiments, the top and bottom sub-cells were interconnected via anintermediate layer, including a band-to-band TJ. In some embodiments,the top and bottom sub-cells were interconnected via an intermediatelayer which may include the RL. A thin metallic or TCO layer acts as arecombination site and can therefore be used as the RL. Hence,appropriate tunneling of the photogenerated charge carriers from onesub-cell to the other via the intermediate layer is essential with alittle electrical/optical loss, ultimately reducing the device cost.

By combining improved light management with a high-quality PVTstructure, about 22.7% conversion efficiency using a heavily dopedhydrogenated nanocrystalline Si (nc-Si:H) tunneling layers on aheterojunction-with-intrinsic-thin-film (HIT) bottom sub-cell, wasreported in the literature. Furthermore, the η of PVT/Si TSC reached toabout 25.2% via insertion of a second nc-Si layer p+/n+TJ. The presentimprovement was achieved which may be attributed to the use of a moreconductive recombination junction (RJ) and the reduction in thereflection losses, which lead to an enhancement in an irradianceresponse.

In some embodiments, to construct monolithic integrated bottom and topsub-cells in a 2T tandem architecture, only a single substrate wasrequired, and fewer layers need to be deposited as compared to themechanically stacked 4T configuration. Moreover, the 2T tandemarchitecture only uses 2 electrodes, one of which is transparent. Hence,the 2T tandem architecture includes a simple structure. Monolithicseries interconnection of the 2T tandem architecture also allows thereduction of the series resistance losses, thus resulting in a higherpractical efficiency potential.

However, the monolithic tandem configuration requires the adaptation ofthe two sub-cells and corresponding interconnection with theintermediate layer (TJ or RL). As the sub-cell restricts the totalcurrent in monolithic 2T tandems with the lower J_(sc), the desiredJ_(sc) value of both the sub-cells should be the same at maximum powerpoint (MPP). The current-matching issue imposes relatively strictrestrictions on the choice of absorbing materials for top and bottomsub-cells. Moreover, some bottom sub-cells (for example, a-Si/c-Si HITcell) are temperature-se, implying that a top PVT cell should beprocessed at low temperatures.

2-Terminal Versus 4-Terminal Tandem Configurations

In 4T tandem devices, the two sub-cells are fabricated individually andmechanically stacked, which can operate independently. Suchconfiguration has the apparent benefit of process straightforwardness.However, using the 4T tandem devices amplifies the cost of the powerelectronics, enhancing the cost of the PV module. On the contrary, the2T tandem architecture employs fewer deposition steps, and only onetransparent electrode was required (compared to 4T tandem architecture)which reduces the manufacturing cost. Thus, parasitic absorption wasalso slashed, which resulted in higher practical efficiency.Additionally, high V_(oc) (sum of two sub-cells) was obtained in the 2TTSC which is advantageous for high voltage devices. Moreover, the 2Ttandem architecture structure offers low resistive loss in photovoltaicsystems. However, the 2T tandem also has certain limitations. Bothsub-cells should deliver nearly the same current during operatingconditions. The sub-cell with the lesser current restricts the outputcurrent. Thus, the optimum E_(g)-range of the top sub-cell is narrowed(1.7 to 1.8 eV). The 2T TSCs are very sensitive to the solar spectrumand illumination intensity deviations. Therefore, a definite scheme isessential for the particular topographical site. Besides, the processingparameters of the top sub-cell fabrication are designated not tointerrupt the performance of the bottom sub-cell. On the other hand, thebottom sub-cell was chosen as an appropriate substrate for the topsub-cell. The theoretical achievable efficiency for a SJ Si solar cellwas about 33%. However, the theoretical limit is extended to about 43%for PVT/Si TSC for the 2T or 4T tandems). A wide gap betweenexperimentally achieved and theoretical efficiency was observed for the2T and 4T tandems devices.

Progress in 2T and 4T PVT/Si Tandems

The combination of PVT and Si in 2T and 4T TSCs has shown significantprogress in recent years with prompt growth in the f. Considerableefforts were made to enhance the properties of materials while reducingthe optical and electrical losses. Example 1: a two-terminalMAPbI₃-based PSC PVT solar cell was fabricated on top of a p-n junctioncrystalline Si solar cell (p⁺⁺-Si/n-Si) along with n⁺⁺-Si tunnel (T)junction. Experimentally measured J-V characteristics provided a η of13.7%.

Example 2: PVT/c-Si 2T TSC was established, which used a homo-junctionc-Si as a bottom sub-cell. TiO_(x) compact and mesoporous layers wereused, which also act as the passivating layers for both the sub-cells.Thus, a high value of V_(oc) was achieved. Furthermore, a texturedstructure was used on the top of the cell, resulting in a 2T TSC with ηof 22.5%.

Example 3: η of 23.6% was achieved for PVT/Si 2T TSC via merging aninfrared-tuned HIT bottom sub-cell with cesium-formamidinium lead halidePVT. LiF antireflection coating (ARC) was used on a top of the cell.

Example 4: Conversion efficiency of 25.5% was achieved for monolithicPVT/Si TSC by employing a textured foil on a top of the PVT/Si TSC TSC.

Example 5: 110 nm thick SiO_(x) interlayers (refractive index=2.6 at 800nm) were inserted between two sub-cells enhancing the J_(sc) (1.4mA/cm²) of HIT bottom sub-cell. A total (top+bottom) J_(sc) of 38.7milliampere per square centimeter (mA/cm²), and η of 25.2% wereachieved.

Example 6: An efficiency of 25% had been initially reported for PVT/Si2T TSC obtained by combining rear-junction SHJ bottom sub-cell withp-i-n PVT top sub-cell. Development of indium doped zinc oxide (IZO)front contact and PVT thickness was allowed for increased J_(sc) valueover 19.5 mA/cm² empowering the η of 26%. The J-V curves under variousillumination spectra revealed that the decline in the J value wasattributed to incomparable illumination somewhat recompensed by a fillfactor (FF). Optical simulation results of the EQE showed that twosignificant damages in J of 3.30 and 4.65 mA/cm² were attributed toparasitic absorption and reflection losses, respectively.

Example 7: 25.4% power conversion efficiency was achieved with a highV_(oc) value of 1.80 V by matching photocurrent (I_(ph)) between top andbottom sub-cells in 2T PVT/Si PSC. The improved photovoltaic performancehad been accomplished by grain engineering of PVT layer throughsynergetic influence of MACl and MAH₂PO₂ additives. Such additives inthe PVT precursor promote grain development and prolong charge carrierlifetime.

Example 8: A PVT layer coating on a pyramidal textured (<1 μm) c-Sibottom sub-cell using a blade coating was proposed. Thenitrogen-assisted blading technique was used to coat a charge transportlayer and the flattening PVT layer. The best V_(oc) value of 1.82 V anda η of 26.0% were achieved. The η could be further improved via defectpassivation of the PVT layer and by minimizing the parasitic absorption.

In some embodiments, the 2T architecture has the advantage of reducingprocessing costs compared to the 4T; however, a direct deposition onto atextured surface of the Si bottom sub-cell is challenging. For example,FIG. 6 shows a simple mechanical stacking of 2T PVT/Si TSC 600. The 2TPVT/Si TSC 600 includes a PVT sub-cell 620 independently fabricated to asilicon bottom cell 640. The PVT sub-cell 620 includes a glass 622,c-TiO2 624, m-TiO2 626, a PVT 628, Spiro-OMeTAD/PTAA 630 and an ITO 632.The PVT 628 was deposited by a solution process. Further, the siliconbottom cell 640 includes front contact and a metal grid. The objectivewas to take the benefits of PSC and textured, c-Si, and SHJ sub-cellsinto 2T PVT/Si PSC. A champion tandem device exhibited an alleviated ηof 25.9%. Such configuration used a larger cell area than the PSC (1.43cm²).

Further, PVT/SHJ 2T TSC was developed. A thick layer of PVT on atextured bottom sub-cell using a solution process. A certified η of25.7% was accomplished via enhancing the depletion width at pyramidaltextures along with passivated PVT layer. The obtained device wasthermally stable and showed no significant η loss at 85 degrees Celsius(° C.) for 400 h.

Further, 4T PVT/Si TSC was developed which delivered η of 27.7%. Variousaliphatic alkylammonium bulky cation layers were deposited on PVT filmresulting in the formation on a surface of 3D PVT of a Ruddlesden-Popper‘quasi-2D’ PVT phase allowing the passivation of the surface defects. Atandem η of 26.2% was reached by combining 1 cm² semitransparent PVTcell with 1 cm² passivated emitter rear locally-diffused solar cell(PERL) silicon cell.

Moreover, a fabrication of an efficient 2T PVT/Si PSCS was established.A PVT layer of large E_(g) employing triple-halide alloys (Cl, Br, andI) was used. For the present process, illumination was needed.Solubility of the Cl was enhanced via substitution of I with the Br.Thus, a charge carrier lifetime and mobility were improved. Anefficiency of 27% had been achieved with an area of 1 cm².

A combination of SHJ bottom sub-cell and PVT top sub-cell in a 2TPVT/SHJ TSC was also established. MA-free FA_(0.75)Cs_(0.25)Pb(I_(0.8)Br_(0.2))₃ PVT layer was used in order to provide an efficientdevice stability, resulting in an enriched absorption close to thebandgap edge along with admirable PVT layer quality. The obtained TSCdelivered an efficient FF of 80%, and certified η of 25.1%. Anefficiency improvement was also observed for the TSC after storing theTSC in dark and under N₂ ambient for more than 5 months.

Further, an outdoor testing was also conducted for the 2T PVT/Si TSCwith η of 25% in hot and sunny weather to elucidate the impact of thetemperature on the PV performance. The temperature of the TSC reached˜60° C. in direct daylight. Moreover, the E_(g) of PVT and Si followopposite trends, which affects the current-matching conditions. Theoptimum E_(g) value of PVT is placed lower than 1.68 eV for theenvironmental temperature of 55° C.

3-Terminal Tandem Architecture

3T TSCs have been proposed and developed as an alternative to 2T and 4Tconfigurations to overcome some limitations mainly related to thecurrent collection or electrical isolation between layers. The 3T tandemarchitecture is compatible with the well-mastered silicon technology. Insome embodiments, the 3T tandem architecture may be obtained bycombining two 2T devices with a middle contact. In some embodiments, the3T tandem architecture may be obtained by combining a 2T top sub-cellwith a 3T bottom sub-cell with one front contact and two interdigitatedback-contact (IBC). Such devices can be demonstrated in gallium arsenidephosphate/silicon germanium (GaAsP/SiGe) structure, amorphous silicon(a-Si) TSC, and polymer-based solar cell. In some embodiments, the 3Ttandem may include PSCs.

In some embodiments, a characterization method of monolithicallyintegrated PVT/Si TSC may be carried out using a 3T measurementarchitecture which includes a high bandgap methylammonium lead halidetop sub-cell and a HIT bottom sub-cell. The 3T measurement architecturemay involve an RL, which also serves as an extra current-collectingelectrode. In some embodiments, the 3T measurement architecture allowedmeasuring the J-V characteristics and external quantum efficiency (EQE)of each sub-cell independently. A PCE of 23.5% was achieved by reducingreflection and absorption losses along with a precise current matching.Moreover, the developed architecture showed an efficient constancy byretaining 97% of the initial η after 100 days under open environments.In some embodiments, a simulation study was conducted to optimize the 3TPVT/silicon tandem structure with the objective of minimizing opticallosses and determining the maximum achievable power conversionefficiency.

In some embodiments, parasitic absorption losses can be avoided when theHTL is located behind the PVT absorber, leading to a significantincrease in total photocurrent density (J_(ph)) from 34.4 to 41.1mA/cm². The simulations have also demonstrated that tandem efficienciesup to 32% can be obtained for np-np configuration with 1000 nm thick PVTlayer in the 3T tandem configuration.

FIG. 7A shows a schematic drawing of a 3T tandem structure 700. The 3Ttandem structure 700 includes layers of components such as LiF 702, IZO704, MeO₃ 706, Spire-OMeTAD 707, PVT 708, SaO₂ 710, ITO 712, a-Si:H (a⁺)714, a-Si:H (i) 716, C—Si (a) 718, Ag 720 and a-Si:H (p⁺) 722, whereinV_(IBC) is the measured voltage of the Ag 720 layer and V_(perovskite)is the measured voltage of the PVT 708 layer. An approach is used tointerconnect a top PVT sub-cell IBC SHJ in a 3T tandem structure. Thepresent structure has eliminated a middle contact in the 3Tconfiguration which may require lateral current collection between topand bottom sub-cells. An effectual charge carrier transportation throughthe entire TSC was therefore obtained. A drift-diffusion simulationrevealed a potential η of ˜27%. Furthermore, the proof-of-concept TSChas reached a combined stabilized η of 17.1% under the MPP condition.However, the measured bias-dependent J-V characteristics have shown aslight mutual dependence of both sub-cells, mainly which can beattributed to the combined series resistance (R_(s)) of bulk andn-emitter of the bottom sub-cell. Discrepancies between simulation andexperimental results can be attributed to the parasitic absorption andreflection in the front-side contact layer stack and the large value ofseries resistance (R_(s)) on the shared n-contact. Also, the shadingwhich was attributed to a metal frame of the front contact of the topsub-cell caused the lowing of the η of TSC.

In some embodiments, 3T PVT-based TSCs include a combination of PVTsolar cells with silicon back-contact solar cells using an internalbarrier (selective band offset barrier) to allow the reduction of thethermalization process by isolating charged current carriers indifferent regions of the device. Numerical modeling was used to studythe performance of such TSCs composed of an n-type IBC bottom sub-celland a PVT top sub-cell with an organic front surface HTL and a standardPVT absorber, which was connected to the IBC by an ETL playing the roleof selective band offset barrier. Preliminary simulation results of thecurrent flow in the whole structure showed the role of the selectiveband offset barrier layer in repelling holes from the top sub-cell andpreventing the corresponding thermalization in the bottom sub-cell. FIG.7B shows a cross-sectional scanning electron microscope (SEM) image ofthe top sub-cell.

Advantages of 3T Tandem Architecture Over 2T and 4T TandemConfigurations

In 2T TSC, top and bottom sub-cells are interconnected in series,implying that the top and bottom sub-cells may have similar currents.However, in a practical device, variation in illumination and theincidence angle during the day created a current mismatch between thesub-cells. However, in 4T TSC, the top and bottom sub-cells weremechanically stacked and independently operated. Thus, the performancewas not limited by the current mismatch between the top and bottomsub-cells. However, the 4T TSC required two extra ITO layers to extractthe current from the sub-cells. The additional layer may inducesignificant parasitic absorption and reflection losses along withadditional resistive losses. In a 3T TSC architecture, the currentmatching between top and bottom sub-cells is not necessary, whichenables an increase in the thickness of the PVT absorbing layer, leadingto higher J_(ph) and hence improved η compared to a 2T TSC architecture.The 3T TSC configuration minimizes mutual electrical influence betweenthe top and bottom sub-cells similar to the 4T architecture as thearchitecture has two separate hole contacts for the top and bottomsub-cells and a common middle contact for electrons. Since the top andbottom sub-cells are operated individually in a monolithicallyintegrated 3T device, the architecture provides a solution tocurrent-mismatch losses in the 2T configuration, and overcomes theoptical losses in the 4T architecture. Moreover, compared to the 2T and4T tandems, the η of the 3T configuration is less sensitive to the E_(g)of the top and bottom sub-cells, which allows a higher flexibility inmaterial choice.

Manufacturing the 2T TSCs has decreased up to 30% environmental impactequivalent to two SJ solar cells. The present decrease can be attributedto the 2T TSCs, which reduced the consumption of glasses. In someembodiments, a comparison between the environmental influences of Si,PSC, and PVT/Si tandem modules was carried out in order to assess globalwarming potential, human toxicity, freshwater eutrophication, freshwaterecotoxicity, and abiotic exhaustion of three PVT/Si TSCs. The PVT/SiTSCs had lesser environmental outcomes than environmental outcomes ofthe Si cells. The structure that presents the best environmentaloutcomes is the PVT/Si tandem using p-n junction Si solar cell (as analternative of HIT), spiro-free PSC, and Al (as a substitute of Ag orAu) as a top electrode.

Technological and Economical

PVT and Si have shown a great potential as compatible partners toachieve high efficiency at little added costs over typical Si solarcells.

Light Management

In some embodiments, PVT/Si TSCs had shown efficient results with anefficiency record of 29.15%. However, to exceed 30% efficiency and gobeyond the theoretical S-Q, one of the prerequisites is the developmentof an effective light management approach that can significantly reduceoptical losses in the tandem structure. In particular, parasiticabsorption related to the use of 2,20,7,70-Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,90-spirobifluorene (Spiro-OMeTAD) (themostly used HTL) and TCO can cause current loss which is a criticalissue for the tandem efficiency. Hence, the parasitic absorption can bemitigated by using an alternative higher mobility materials such as IZOand hydrogenated indium oxide (H—InO_(x)). Additional losses can takeplace which can be attributed to optical interference effects, which canoccur in a PVT sub-cell and to reflection at interfaces, thuscontributing to the increase of current loss in the tandem structure.Hence, enhancing light trapping and minimizing reflections of a frontsurface to improve overall light harvesting in a tandem stack is anefficient method. The enhancement of a near response at a band edge, andthe removal of the reflection losses can be achieved by surfacetexturing of c-Si wafers, and by avoiding the use of materials with lowrefractive indexes between the sub-cells. The vapor deposition processmay be employed to deposit the PVT layers as the process may adapt tothe strident topographies of textures of the bottom sub-cells.

Economic Prospects

In some embodiments, a bottom-up cost and uncertainty model was used todetermine levelized cost of energy (LCOE) of 2T PVT/Si TSCs. Expensivecharge transport materials such as [6,6]-phynyl-C61-butyricacid methylester (PCBM), PTAA, and Spiro-OMeOTAD may be replaced withcost-effective charge-transporting layers to minimize the LCOE of PVT/SiTSCs. Furthermore, the LCOE was further lessened by reducing the lossesof the materials using a spin coating technique. In some embodiments,coating techniques such as screen-printing for the PVT and chargetransporting layer formation can also be employed. In some embodiments,the cost was further reduced by using cost-effective CZ wafers in placeof Float-zone (FZ) wafers. In some embodiments, the cost was reduced byusing p-type wafers as an alternative of an n-type Si wafer. In someembodiments, the cost was reduced by using Si homo-junction instead ofhetero-junction.

In some embodiments, in PVT/Si TSC, a bottom sub-cell contributes lessthan half of the total η of the TSC. Therefore, replacing c-Si withme-Si has less impact on the device performance, however, LCOE can begreatly reduced. Thus, the LCOE of PVT/Si TSC was further lessened byusing the me-Si wafer along with cost-effective processing methodology,including wafer synthesis techniques, kerfless, and ingot casting. Insome embodiment, 4T TSC configuration is efficient to replace expansivec-Si with me-Si bottom sub-cell as PVT top sub-cell is first fabricatedindependently and then mechanically stacked with the bottom sub-cell.

Proposed TSC Structure

FIG. 8 shows a schematic diagram of an exemplary PVT/Si TSC structure800 on an n-type Si wafer (c-Si or me-Si) as a bottom sub-cell. ThePVT/Si TSC structure 800 includes a front transparent contact (FTC) 802,a HTL layer 804, a PVT layer 806, a TCO layer 808, a gridded metalcontact 810, an ETL (p⁺ doped and surface modified Si) layer 812, ann-Si layer 814, an n⁺⁺-Si layer 816, a back contact layer 818. Electrons(e⁻) move from the PVT layer 806 towards the ETL (p⁺ doped and surfacemodified Si) layer 812. Furthermore, protons (h⁺) move from the PVTlayer 806 to the HTL layer 804. With the proposed structure of theperovskite/silicon 3T tandem structures using ‘middle contact’configuration, over 30% efficiency was feasible. Moreover, theefficiency was further improved using selective contact, and factorssuch as low-cost Si (CZ c-Si or me-Si) solar cells used as bottomsub-cells can reduce the cost of tandem solar cells. Conduction andvalence band edge at a front surface may be tuned via a surfacemodification. A modified silicon surface may be used as an ETL. Buriedcontact may be used for the current collection, which may further reducethe shadow losses. TCO (FTO or ITO) may be used as a recombination layeras well as middle contact. SiOx or Al₂O₃ layer can be used as apassivating layer. Bandgap and thickness may be flexible of perovskitetop-cell, which can be attributed to no current mismatch. An inorganicperovskite layer may also be used. A highly diffused n⁺⁺ junction may beformed on the rear surface of Si.

The PVT/Si TSC structure 800 provides advantages such as omission ofseveral processing steps, parasitic absorption losses (attributed tointerface layers) can be reduced; utilization of wide range photons forthe current generation, omission of various interfacial layers leadingto the reduction of the cost of the tandem cell; involvement of low-costbottom sub-cells (CZ c-Si and me-Si) also cut the cost of the tandemcell; the tandem cell includes a low environmental impact, addition ofinorganic charge transporting layer has decreased the cost and hasincreased the stability of the tandem device; conversion efficiency ofthe TSC can be enhanced to >30%, thereby decreasing the overall cost ofthe tandem cell. Furthermore, direct entry of charge carriers to the Sileads to a reduction of resistive loss. Moreover, the current density ofthe tandem cell can also be enhanced. Surface modification of the Sienhances the V_(oc). Overall V_(oc) of the tandem cell is alsoincreased. Various environmental conditions such as illumination andtemperature maximize the output power (P_(m)). The patterned metalliccontact of the bottom sub-cell along with the TCO layer can be used asmiddle contact for the current collection. In some embodiments,thickness or bandgap of a top sub-cell can be optimized.

Methodology for n-Type Si Wafer and 3T Perovskite/Silicon Tandem CellFabrication

FIG. 9 is a schematic diagram of proposed monolithic PVT/Si 3T tandemdevice 900 for n-type wafer-based homo-junction silicon solar cell alongwith upconverters in point contact configuration. The monolithic PVT/Si3T tandem device 900 includes an ARC layer 902, a first TCO layer 904, aHTL layer 906, a PVT layer 908, a buried contact 910, a second TCO layer912, an ETL layer 914, an n-Si layer 916, an n⁺-Si 918, and a backcontact layer 920, with the 3 terminals A-C. A polished or texturedn-type Si wafer of resistivity 1-10 Ω-cm, and thickness of 200 μm wasused as a starting material. The wafers were cleaned using a RadioCorporation of America (RCA) cleaning process. SiO₂ masks were grown viasputtering, atomic layer deposition, or thermal evaporation. Patternedwindows were opened in the SiO₂ layer for point contact formation usinglithography or nanolithography. The SiO₂ layer was removed in dilutehydrofluoric acid (HF) followed by standard cleaning. Front and backsideof the wafer were simultaneously diffused using B-source and P-source tocreate p⁺/n junction and n/n⁺ (low-high) junction. A thin, poroussilicon (PS) layer was grown using chemical or electrochemical routes.Pattern metallic contacts were prepared on a front surface Si bottomsub-cell. Full metallic contact was prepared on a rare side of thebottom sub-cell. SiO₂ or Al₂O₃ layer of about ˜10 nm was deposited viaatomic layer deposition (ALD) of sputtering techniques on PS. The PVTlayer was deposited via a chemical vapor deposition (CVD), evaporation,or solution process. Further, HTM (NiO, NiO:Cu, or WO₃) was depositedvia spin coating. Front contact of TCO (such as ITO or IZO) wasdeposited via thermal evaporation or sputtering.

FIG. 10 shows a schematic diagram of an exemplary PVT/Si TSC structure1000 on a p-type Si (c-Si or me-Si) wafer as a bottom sub-cell. Thepresent structure includes a front contact (TCO) 1002, a HTL layer 1004,a PVT layer 1006, a TCO layer 1008, a gridded metal contact 1010, an ETL(p+-PS) layer 1012, a c-Si (p) layer 2014, a c-Si (n+) layer 2016, aback contact layer 2018. Electrons (e⁻) move from the PVT layer 1006 tothe ETL (p+-PS) layer 1012, and the protons (h⁺) move from the PVT layer1006 to the HTL layer 1004. The PVT/Si TSC structure on p-type Si (c-Sior me-Si) wafer has similar advantages as that of the PVT/Si TSCstructure on the an-type Si wafer (c-Si or me-Si) and similar advantageswith few changes, such as front and backside of the wafer weresimultaneously diffused using B-source, and P-source to create n+/pjunction and p/p+ (low-high) junction. Furthermore, long-wavelengthphotons were absorbed near p-n junction. Collection efficiency wasincreased.

FIG. 11 shows a schematic diagram of a proposed monolithic PVT/Si 3Ttandem device 1100 for p-type wafer-based homo-junction silicon solarcell along with upconverters in point contact configuration. Themonolithic PVT/Si 3T tandem device 1100 includes an ARC layer 1102, afirst TCO layer 1104, a HTL layer 1106, a PVT layer 1108, a buriedcontact 1110, a second TCO layer 1112, an ETL layer 1114, a p-Si layer1116, an n⁺-Si 1118, and a back contact layer 1120 and terminals A-C.

The present disclosure provides an efficient and ergonomic solar cell.The PVTs provides flexibility, wide area, lightweight, andcost-effective solution processed PV technology. The PVTs have uniqueoptoelectrical properties including strong optical absorption, lowexciton binding energies, long diffusion lengths (>1 μm), large carrierlifetimes, high charge carrier mobility, low trap densities, and highdefect tolerance. The optoelectrical properties of the PVT active layercan be tailored through variations of organic cation and halide anion.The capability of the PVT materials makes the PSCs an efficientcandidate for the front sub-cells in the tandem architecture.

Obviously, numerous modifications and variations of the presentdisclosure are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

The invention claimed is:
 1. A three-tandem (3T) perovskite/silicon(PVT)-based tandem solar cell (TSC), comprising: an antireflectioncoating (ARC); a first transparent conductive oxide layer (TCO), whereinthe ARC is adjacent and above the TCO; a hole transport layer (HTL),wherein the TCO is adjacent and above the HTL; a perovskite (PVT) layer,wherein the HTL is adjacent and above the PVT layer; a secondtransparent conductive oxide layer (TCO), wherein the PVT layer isadjacent and above the second TCO; an electron transport layer (ETL)comprising a porous silicon surface, wherein the second TCO layer isadjacent and above the ETL; a plurality of buried contacts, wherein theburied contacts comprise silicon nanowires and are disposed in and passthrough the ETL into the second TCO and into a p-type Si layer; a p-typewafer-based homo-junction silicon solar cell with the buried contactsbeing in point contact configuration with the ETL, wherein the ETL isadjacent and above the p-type silicon solar cell; a n⁺ silicon layer,wherein the p-type silicon solar cell is adjacent and above the n⁺silicon layer; a back contact layer, wherein the n⁺ silicon layer isadjacent and above the back contact layer, which uses an internalthermal barrier; and wherein the three-tandem (3T) perovskite/silicon(PVT)-based tandem solar cell (TSC) has a top sub-cell, a bottomsub-cell and a middle contact-based tandem; wherein the top sub-cellincludes the PVT layer; the bottom sub-cell includes the silicon solarcell; and the middle contact-based tandem includes the second TCO layerto be used as the middle contact-based tandem, as well as arecombination layer for current collection; and a conduction and avalence band edge are employed at a front surface of the ETL; the poroussilicon surface is directly grown on a surface of the silicon solarcell; and the band edge is tuned by varying a porosity of the poroussilicon surface.
 2. The solar cell of claim 1, wherein the bottomsub-cell comprises a CZ crystalline silicon (CZ c-Si).
 3. The solar cellof claim 1, wherein the bottom sub-cell comprises a multi-crystallinesilicon (mc-Si).
 4. The solar cell of claim 1, wherein the first andsecond TCO layers are fluorine or indium doped tin oxide (FTO or ITO).5. The solar cell of claim 1, wherein the first TCO layer is ahydrogenated indium oxide.
 6. The solar cell of claim 1, wherein the topsub-cell includes an organic or inorganic perovskite.
 7. The solar cellof claim 1, wherein the PVT layer comprises one or more perovskiteshaving a formula ABX₃; wherein A is a monovalent cation selected fromthe group consisting of methylammonium (MA⁺:CH₃NH₃ ⁺), formamidinium(FA⁺:HC(NH₂)₂ ⁺), cesium (Cs⁺), and rubidium (Rb⁺); B is a divalentmetal cation selected from the group consisting of Pb²⁺, Sn²⁺, and Ge²⁺,and X is a halide anion selected from the group consisting of Cl⁻, Br⁻,and I⁻.
 8. The solar cell of claim 1, wherein the porous silicon surfaceuses an optical band-edge shifting property to directly extractlight-generated charge carriers.
 9. The solar cell of claim 1, whereinthe bottom sub-cell contributes less than half of a total efficiency ofthe TSC.
 10. The solar cell of claim 1, wherein the second TCO layer hasa gridded metal contact disposed adjacent to a surface of the second TCOlayer.
 11. The solar cell of claim 1, wherein the p-type silicon solarcell has a resistivity of from 1 (ohm-centimeters) Ω-cm to 10 Ω-cm. 12.The solar cell of claim 1, wherein the p-type silicon solar cell has athickness of from 175 micrometers (μm) to 225 μm.
 13. The solar cell ofclaim 1, further comprising at least one of a SiO₂ layer or Al₂O₃ layeron the porous silicon surface.
 14. The solar cell of claim 13, whereinthe SiO₂ or Al₂O₃ layer is at most 10 nanometers (nm).
 15. The solarcell of claim 1, wherein the HTL comprises a hole-transmitting material(HTM).
 16. The solar cell of claim 15, wherein the HTM is selected fromthe group consisting of NiO, NiO:Cu, and WO₃.